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Micro FM Transmitter 

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Micro FM Transmitter 

Written By: Sean Michael Ragan 



3/8" drill bit (1) 

Copper wire (4 inches) 

Audio signal source (1) 


1/4" TRS jack (1) 

Bolt or machine screw (1) 

Onlv tip and shield connections are 

For use as a mandrel in tormina the coil. 


FM radio (1) 

Batterv holder clips (1) 

Helping hands (1) 



Needle Nose Pliers (1) 

or use perfboard and jumpers. 

Scissors (1) 


Double-sided foam tape (1.5") 

Screwdriver (1) 
To fit trim capacitor. 
Side-cuttina pliers (1) 



use your imaainationi Mine was 
salvaaed from a thrift-store diaital clock. 
Ajellv jar works areat too! 

Soldering Iron and rosin core solder (1) 

Battery (1) 

Wire cutter/stripper (1) 


Mini trim capacitor (1) 

hand-held electric drill (1) 


Ceramic capacitor M) 


Ceramic capacitor 12) 


NPN silicon transistor (1) 


Electrolytic capacitor (1) 


Metal film resistor (1) 

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Pagel of 8 

Micro FM Transmitter 

Metal film resistor (1) 
Metal film resistor (1) 
Multistrand hookup wire (8") 
Bubble wrap (6 sq in) 


This circuit is commonly credited to Japanese multimedia artist Tetsuo Kogawa . It takes 
audio input through a 1/4" phono jack and, constructed as shown, without the optional 
antenna connections, will broadcast an FM radio signal about 30 feet. 

This is the standard model of Mr. Kogawa's simplest FM transmitter , which is slightly more 
complex than his most basic design in that it includes a trim capacitor to adjust the 
transmitting frequency. It can be powered by a 9V battery and uses a hand-turned copper 

I'm using the PCB and parts from Sonodrome's old kit , but the circuit is extraordinarily 
simple and could be built on perfboard or on a panel almost as easily. Sonodrome provides 
free PCB art if you want to etch your own board. Kogawa himself provides instructions for 
building the transmitter on an unetched copper-clad panel. 

Step 1 — Prep case 

• Disassemble your case. 

• Mark and drill a 3/8" hole, in an appropriate location, for the TRS jack. 

• At this time, you may also want to drill mounting holes for a power switch and/or a 
power jack, if you choose to use an external supply. 


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Page 2 of 8 

Micro FM Transmitter 

Step 2 — Prep jack 

• Strip about 1/2" from each end of two 4" pieces of 24 AWG hookup wire. 

• Tin the stripped ends. 

• Solder one end of one lead to the front leg of the 1/4" TRS jack, and one end of the other 
lead to the back leg. 

Step 3 — Form coil 

• Take a piece of 19 AWG enameled copper wire, about 4" long, and wind at least four turns 
about the threads of a 1/4-20 bolt or machine screw. 

• Rotate the bolt counterclockwise to unscrew the coil from the thread. 

• You want a total of four turns in the coil. Use small pliers to bend two legs down, as 
shown, and side-cutting pliers to clip them to about 1" long. 

• The mounting holes for the coil legs should be 12mm apart on the surface of the 
PCB. The act of installing the coil on the board should stretch it to the correct 
length, but you may have to tweak it a bit with pliers or a screwdriver to make sure the 
rate of coiling is even between the two legs. 

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Page 3 of 8 

Micro FM Transmitter 

Step 4 — Install components 

• Bend and slip the component leads into the correct holes in the PCB. 

• Carefully note and verify the correct orientation of the electrolytic capacitor and 
wire leads. 

• Bend the leads on the solder side of the board to temporarily secure the components in 
place. Clip them to about 1/4" to open up a bit of room to solder in. 


Step 5 — Solder components 

• Clip the PCB, trace-side up, into your helping hands, or otherwise secure it, in a level 
horizontal orientation. 

• As always when soldering, be sure to work with plenty of ventilation and avoid 
inhaling fumes. 

• Flux, heat, and solder each lead in place. 

• Once the solder has cooled, clip any protruding leads with side or end-cutting pliers 


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Page 4 of 8 

Micro FM Transmitter 

Step 6 — Mount PCB 

• Mounting details will naturally vary with the case you choose. This foam-tape method 
worked great for my salvaged clock case, but your mileage may vary. 

• Attach a strip of double-sided foam tape to the featureless corners of the PCB, on the 
trace side, as shown. Each strip is about 1/4 x 3/4". 

• Do not attach tape directly over the traces, or you may damage them if you ever 
have to remove the tape. 

• Remove the backing from the foam tape strips on the PCB. 

• Making sure the leads are first correctly positioned, carefully orient the PCB and press it 
into place. 


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Page 5 of 8 

Micro FM Transmitter 

Step 7 — Tune circuit 

• Connect a fresh 9V battery to the 
battery clip, and an audio signal to 
the TRS jack. I used the 
headphone jack of my laptop for an 
audio source. 

• Turn on your FM radio, and scan 
around looking for your signal. I 
found mine at around 99.8 MHz. It 
may be quite static-y and noisy at 

• Use a small screwdriver to adjust 
the trim capacitor, as shown, until 
your signal comes through loud and 

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Page 6 of 8 

Micro FM Transmitter 

Step 8 — Assemble 

• Remove the washer and nut from the phono jack and thread it, from inside the case, 
through the hole you drilled in Step 1. 

• Put the washer over the threads and tighten the nut down from outside the case, to secure 
the jack. Finger-tight is fine. 

• Attach a 9V battery to the clip, pad it with a scrap of bubble wrap, and stuff it in the case 
before sealing up. 

• I'm going to to modify my transmitter with a jack for an external regulated power 
supply. You may want to do the same, or at least add a power switch between the 
battery and the board. But for testing purposes, this set-up will suffice. 


Step 9 — Use it! 


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• Turn your radio on again, pick it up, 
and walk away from the bench 'til 
the signal fails. Mine was loud and 
clear to about 30 feet. 

• Depending on where you 
live, operating even a very 
short range FM transmitter like 
this, without a license, may conflict 
with applicable laws and/or 
regulations. Be sure to investigate 
carefully before turning it on, and 
err on the side of caution if in 

Page 7 of 8 

Micro FM Transmitter 

st generated on 2012-10-31 11:34:29 PM. 

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